Simple Version: http://gw2timer.com/tile
Overlay App: http://gw2timer.com/navi
The timers are only a guideline based on previous spawn research, always use Levvi's Device to tell what the current set of creatures are, which may be timed randomly.
The Bloodstone Harvest achievement released on 2016-08-09 rewards you with a smoky mask skin [&AgHPNQEA] after killing 5 different legendary creatures and collecting 10 bloodstone slivers.
To know what creatures are in the spawn time window (rotation), get Levvi's Device from a corpse in Verdant Brink [&BPsHAAA=] (Hold Ctrl when you're there to see the name on screen). To use Levvi's Device, stand on and face a ledge or a rock so there is space below you, move your inventory window to a corner, double click the device, then zoom in to see the mini creatures under the red smoke. If you see only a single pink puff, that means no creature is in the current spawn window. If you see a red skull over the mini creature, that creature has died in the map instance you're in, so you can either go to a different map that spawns that type of creature, or join another player in a different instance of the same map.
To spawn the legendary creature, kill regular mobs (about 5 to 30 or more) of that creature type in those maps; a special case is in Gendarran Fields, where you kill brown bears instead to spawn the Arctodus. If you're lucky the legendary creature will spawn directly on the regular mob you killed.
In the LFG tool, join a likeminded Central Tyria squad to make the search much easier. You can join players in other instances by first being in the same map as them then right clicking their name for the "Join in" option. Shark takes a long time to kill, Wyvern a bit of time, and other creatures may melt fast. So when you just mapped in, ask the players to slow down their DPS. Note that you only need to tag the creature for a few thousand HP to get credit, so you can stop attacking to let other players arrive in time.
Timings and Rotation
The three rotations' pattern: Wyvern --> Moa+Devourer --> Arctodus+Shark, repeat.
A rotation always lasts 1 hour. A new rotation may be introduced exactly 30 minutes after the current began, so it is possible to concurrently have Wyvern with Moa+Dev, Moa+Dev with Arc+Shark, or Arc+Shark with Wyvern. Otherwise after the current rotation finishes, there will be a random break that can last as short as a few seconds up to 40 minutes (from my observations) until a new rotation begins. This cycle as a whole restarts at every daily reset time, starting with Wyvern in the window between 00:00 UTC to 01:00 UTC (game clock server time).
- The spawn windows were determined by what creatures were shown on Levvi's Device, as determined in a combined log of August 10 and August 11, with a sampling of every 5 or 10 minutes, then "categorized" into :00, :15, :30, or :45 time windows.
- The timers on the website are split into divisions of 15 minutes in the clock's hour, for the sake of convenience.
- The creatures may show up on Levvi's Device at any minute during that time chunk.
- To be safe, the timer always show what the "expected" creatures will be, even if it's "break time".